A gas sensor is one kind of sensor device for detecting gas concentration. Generally, gas sensors are utilized to monitor for leakage of toxic gas, environmentally harmful gas, combustible gas, and explosive gas for early warning in process control. Gas sensors are also used for home applications. For instance, a gas sensor can be used to detect fuel gas and carbon monoxide leaks to protect people in the house. In addition, an alcohol sensor can be used to detect breath-alcohol concentration in persons suspected of driving under the influence of alcohol. In addition, gas sensors can be utilized in vehicles such as automobiles to monitor engine combustion efficiency and exhaust gas emissions. Currently, rapidly increasing numbers of home, automobile, and wireless network applications require increasing numbers of gas sensors. According to a global survey conducted in 2001, the global market of gas sensors was about 2.3 billion dollars and the report estimated that with an annual mean growth rate of about 5.9%, the global market would reach 3.5 billion dollars.
Gas sensors are categorized by the gas detected or by the type of sensor. There are many different types of gas sensor including electrochemical, solid-state electrolyte, optical, metal oxide semiconductor, and others. Early gas sensor used a liquid electrochemical mechanism. The early gas sensor has been largely replaced by solid-state electrolyte gas sensors, which are more easily miniaturized. However, optical gas sensors require complicated optical systems. Current solid-state electrolyte gas sensor designs include catalyst with flame, metal oxide semiconductor, solid-state ion-conductor, and field-effect transistors.
The first commercial semiconductor gas sensor was produced by Figaro Eng, Inc. in 1967. This Taguchi-type gas sensor was utilized for detecting alcohol, fuel, and other gas. The Taguchi-type gas sensor is composed of a metal heating coil (such as a platinum coil), a metal oxide semiconductor sensing material coating on the metal heating coil, and sensing electrodes disposed at two terminals of the sensing material. The metal heating coil coated with the sensing material and the sensing electrodes are cantilevered in a TO can to form a tubular sensing structure.
Researchers have developed another semiconductor gas sensor manufactured using thick film technique. The thick-film gas sensor is imprinted by stencils. Because the stencil can imprint the heater circuit, gas sensing circuit, and sensing material on the ceramic plate, it is possible to manufacture in batches.
Since most semiconductor gas sensors are required to operate under predetermined high temperature to achieve consistency, sensitivity, and reactivity, the heater circuit provides a high temperature micro-environment for operation of the gas sensor. Although both the Taguchi-type gas sensor and the stencil-imprinting gas sensor use the metal conducting line to cantilever the sensing material in the package to avoid unnecessary heat emission, power consumption of both of these gas sensors is still greater than 1 watt. For the purposes of miniaturizing components and reducing power consumption, in the 1980's, researchers developed the Micro Electro Mechanical Systems (MEMS) technique to fabricate micro gas sensors. The micro gas sensor is fabricated on a wafer substrate by a process similar to the semiconductor process, allowing the micro gas sensor to be manufactured in batches and easily miniaturized on a silicon chip with signal amplifier.
Since the micro gas sensor is fabricated on the silicon wafer, which has a thermal conductivity greater than that of ceramic material, the micro gas sensor requires more consideration of the thermal conductive paths to minimize heat emission on the silicon substrate. Thus, most types of MEMS gas sensors are produced by etching a silicon substrate to form a very thin film as a gas sensing region and to reduce power consumption. There are two types of thin film structure of the gas sensors including a close-membrane type and a suspended-film type. The close-membrane type gas sensor has a gas sensing region in an enclosed film. The suspended-film type has its gas sensing region on a thin film suspended by several cantilevered beams. In the structural view, the heat emission paths are limited to the cantilevered beam in the suspended-film type gas sensor; therefore, the suspended-film type gas sensor has much lower power consumption, but its thin film structure is vulnerable.
The films of common close-membrane type gas sensors are formed through deposition of dielectric such as silicon oxide, silicon nitride, and so on. The heater circuit and temperature sensing circuit are formed inside of the film of the close-membrane package. The gas sensing electrodes and sensing material are disposed on the film. Since the film includes many layers with different thermal expansion coefficients, the inner stress of the film due to unequal thermal expansion coefficients will distort or even crack the sensing film. In addition, since the temperature of the gas sensor fluctuates between high operating temperature and room temperature, the sensing film of the close-membrane type gas sensor has several layers that are vulnerable due to different thermal expansion coefficients among these layers. For suspended-film type gas sensors, there is a challenge in integrating the sensing material and the sensing film. Under high operating temperatures, the metal heater circuit can be destroyed due to metal deterioration.
U.S. Pat. No. 6,161,421 discloses a gas sensor for detecting alcohol including a cantilever bridge composed of silicon carbide, SiC as a sensing film and a heater circuit. The SiC film includes sensing electrodes and the sensing material for detecting alcohol in the air.
U.S. Pat. Nos. 7,495,300 and 7,157,054 disclose MEMS gas sensors, both of which are formed on the silicon on insulator, Silicon-on-Insulator (SOI) wafer substrate. Both of these gas sensors can be fabricated using standard CMOS or BiCMOS processes incorporating post-CMOS MEMS process. One characteristic of these gas sensors is that the amplifier circuit and the operating circuit are fabricated on the same chip. The difference between the above patents is that U.S. Pat. No. 7,495,300 utilizes tungsten in the CMOS process to form the heater circuit in the micro-gas sensor, whereas U.S. Pat. No. 7,495,300 utilizes polysilicon in the CMOS process to form the heater circuit in the gas sensor. Such film is vulnerable due to the inner stress from different thermal expansion coefficients among the layers in the film.
J W Gardner et al, Journal of Physics: Conference Series, 15, pp. 27-32, 2005, proposed an integrated micro-gas sensor in the SOI wafer substrate. The micro-gas sensor can integrate with a MOSFET circuit in the same chip. The heater circuit of the gas sensor is composed of a monosilicon layer in SOI wafer substrate, wherein the power consumption of the monosilicon heater circuit is less than that of polysilicon heater circuit or metal heater circuit in a high temperature operating situation. However, the sensing film of the gas sensor is also vulnerable in high temperature operating situations due to its layers including different materials with different thermal expansion coefficients.
Gwiy-Sang Chung, Metals and Materials International, Vol. 8, No. 4, pp. 347-351, 2002, proposed a micro-gas sensor in the SOI wafer substrate. One characteristic of the gas sensor manufacturing process is the etching of the sensing film to form a groove and then to insert silicon oxide into the groove. The paper presented that the maximum heating temperature of the gas sensor without any groove is 280; however, the maximum heating temperature of the gas sensor with ten rounds of the above-mentioned grooves is 580.